This invention relates to an optical communication system and method configured for the distribution of a key using quantum cryptography.
The purpose of cryptography is to exchange messages in perfect privacy between two users, conventionally known as Alice and Bob. Cryptography methods often use a publicly announced encrypting and decrypting algorithm; the confidentiality of the information relies entirely on a key which must be used as an input to the decrypting algorithm for decrypting the received messages.
The key usually consists of a randomly chosen, sufficiently long string of bits. Once the key is established, subsequent messages can be transmitted safely over a public channel. However, two users wanting to communicate must at a certain stage use a secure channel to share the key. With conventional key transmission methods, which can be subject to passive monitoring by an eavesdropper, it is impossible to transmit a certifiably secret key, and cumbersome physical security measures are required. However, secure key distribution is possible using quantum techniques. In quantum cryptography, the key is exchanged through a quantum channel. Its security is based on the principles of quantum mechanics which state that any measurement of a suitably chosen quantum system will inevitably modify the quantum state of this system. Therefore, an eavesdropper, Eve, might get information out of a quantum channel by performing a measurement, but the legitimate users will detect her and hence not use the key. In practice the quantum system may be a single photon propagating through an optical fiber, and the key can be encoded by its polarization or by its phase, as proposed by Ch. Bennett and G. Brassard in  less than  less than Quantum Cryptography: Public key distribution and coin tossing greater than  greater than , Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984).
Interferometric quantum key distribution systems are usually based on a double Mach-Zehnder interferometer, one side for Alice and one for Bob (see FIG. 1). These interferometers implement time-multiplexing, as both interfering pulses follow the same path between Alice and Bob, with some time delay. However, the pulses follow different paths within both Alice""s and Bob""s interferometers. In order to obtain a good interference, both users therefore need to have identical interferometers, with the same coupling ratios in each arm and the same path lengths, and also need to keep them stable within a few tens of nanometers during a transmission. Therefore, one interferometer has to be adjusted to the other every few seconds to compensate thermal drifts. Moreover, since optical components like phase modulators are polarization dependent, polarization control is necessary both in the transmission line and within each interferometer. In polarization-based systems, the polarization has to be maintained stable over tens of kilometers, in order to keep aligned the polarizers at Alice""s and Bob""s. Obviously, this is inconvenient for practical applications.
One technical problem the invention wishes to solve is thus to find an improved device and an improved method of quantum cryptography.
According to various aspects of the present invention, these improvements follow from the features of the characterizing part of the independent claims.
More specifically, these improvements follow from one system in which the interfering pulses run over the same branches of the interferometer, but in another sequence, so that they are delayed in time when they run over said quantum channel.
The system of the invention thus allow a system to be built which needs no alignment or balancing of the interferometer. Using the system of the invention, Alice and Bob can thus exchange information, e.g. a cryptographic key, through a standard telecommunication channel. The users at both ends of a channel only need to plug in the inventive sending/receiving station and the inventive key encoding station, synchronize their signals, and start the exchange.
According to another aspect of the present invention, cancellation of polarization effects is obtained by using Faraday mirrors at the end of the fibers.